I am doing a study to see if a dynamic analysis can be done on the compressor discharge header. This header is connected to discharge piping of three compressors. It is outside of the compressor building. The compressor vendor has done acoustic analysis for the compressor piping within the compressor building.

My questions are:

1. The vendor only supplied shaking forces for the piping inside of the compressor building. How can I calculate the shaking force of the header? If I use the harmonic analysis for the header, should I apply the shaking force for each elbow at the same time?

2. How to consider the effects of the vibraion of the compressors and dischrage piping on the header?

Thanks,

Hello my friend,

Typically you should model away from the compressor to the first large change in volume that will reflect all incoming pulsations. This might be:

a) Knockout drum
b) Offgas column connection
c) Pressure letdown valve
d) Well head
e) Long Pipeline

The distance to be modeled into the pipeline is a function of what you desire to study. The interaction of compressors at facilities more than 5km apart have been witnessed, but typically only the piping in the vicinity of the compressor is of concern.

Is there vibration problem in the firld now? If so, the majority of field problems are due to either:

1) Incorrect or non-existent pulsation suppression
2) Poor supporting
3) Matching of mechanical and acoustic natural frequencies at a machine harmonic.

In any of these cases, the measured significant response (displacement of the pipe system) occurs at one of the harmonics of the system running speed, and the mode of vibration is easy to predict with CAESAR II that calculates the natural frequencies and mode shapes of pipe and structural
systems.

So, if you could give more details of existing problems and piping routes, then I would be able to help and investigate it more.

Regards,
Farhad Salehi

Thanks Farhad.

This is new piping. The client asked us to do a modeling on the header to see if there is significant vibration. Any suggestions on how to model it?

Below is a picture of the header. The main header is 20". The discharge piping of each compressor is 8".

Dear SSW,

Note that, “Design Phase” analysis is typically more difficult because you must make sure that all possible scenarios are simulated. All potential excitation frequencies and gas conditions must be investigated, and all the applicable rules of API 618 must be satisfied. Excessive pulsations can
prevent the effective running of the machine, can damage valves and other mechanical parts, and can produce large shaking forces in the piping. Climate and local ambient air changes can often result in variations in the speed of sound of up to 15%. Typically, you should analyze the variety of gas and ambient conditions as separate load cases. The worst of these will then be used for the mechanical response study.

For your case above, I recommend the following analysis of that part of piping system which vendor has simulated. In my opinion, 20" header acts like a large surge vessel that can reduce pulsation level, of course it's only my guess.

1- Add all structural portals, T-posts, frames, etc. and perform Modal analysis to calculate natural frequencies of the piping system. Compressor will produce shaking forces not only at the basic compressor frequency but also at the higher harmonics. In order to avoid mechanical resonance sufficient separation has to exist between mechanical natural frequencies and the compressor harmonics, i.e. about 20%.

2- Perform Harmonic analysis and apply shaking forces supplied by vendor with phase shift angles between 3 compressors, if any. The resulting pipe, supports and structural stresses are also compared to the fatigue curves for the applicable materials (No. of cycles = 86400 x RPM/60 x days). In most cases, pipe supports and bolts fail sooner than pipe components.

I hope it helps.